Power amplifier for amplification of an input signal into an output signal

ABSTRACT

A power amplifier comprising a first, second and third sub-amplifier for amplification of an input signal into an output signal is disclosed. The sub-amplifiers are connected to an input network and an output network. The output network comprises a first, second and third transmission line connected to the first, second and third sub-amplifier, respectively. A difference in electrical length between the first, second and third transmission lines is an integer number of quarter-wavelengths of a center frequency of the power amplifier. A first, second and third electrical length includes the first, second and third transmission line, respectively. A longest one of the electrical lengths is at least a multiple of quarter-wavelengths of the center frequency. Furthermore, a radio network node, comprising the power amplifier, and a user equipment, comprising the power amplifier, are disclosed.

TECHNICAL FIELD

Embodiments herein relate to wireless communication systems, such astelecommunication systems. In particular, a power amplifier foramplification of an input signal into an output signal is disclosed.Furthermore, a radio network node, comprising the power amplifier, and auser equipment, comprising the power amplifier, are disclosed.

BACKGROUND

Power amplifiers are widely used in communication systems, for examplein radio base stations and cellular phones of a cellular radio network.In such cellular radio network, power amplifiers typically amplifysignals of high frequencies for providing a radio transmission signal. Aconsideration in the design of power amplifiers is the efficiencythereof. High efficiency is generally desirable so as to reduce theamount of power that is dissipated as heat. Moreover, in manyapplications, such as in a satellite or a cellular phone, the amount ofpower that is available may be limited due to powering by a battery,included in e.g. the satellite. An increase in efficiency of the poweramplifier would allow an increase of operational time between chargingof the battery.

A conventional Power Amplifier (PA), such as class B, AB, F, has a fixedRadio Frequency (RF) load resistance and a fixed voltage supply. Class Bor AB bias causes the output current to have a form close to that of apulse train of half wave rectified sinusoid current pulses. The DirectCurrent (DC), and hence DC power, is largely proportional to the RFoutput current amplitude, and voltage. The output power, however, isproportional to the RF output current squared. An efficiency of theconventional power amplifier, i.e. output power divided by DC power, istherefore also proportional to the output amplitude. The averageefficiency is consequentially low when amplifying signals that onaverage have a low output amplitude, or power, compared to the maximumrequired output amplitude.

Known RF power amplifiers include both Doherty and Chireix type poweramplifiers. These kinds of RF PAs are generally more efficient than theconventional amplifier described above for amplitude-modulated signalswith high Peak-to-Average Ratio (PAR), since they have a lower averagesum of output currents from the transistors. Reduced average outputcurrent means high average efficiency.

The reduced average output current is obtained by using two transistorsthat influence each other's output voltages and currents through areactive output network, which is coupled to a load. By driving theconstituent transistors with the right amplitudes and phases, the sum ofRF output currents is reduced at all levels except the maximum. Also forthese amplifiers the RF voltage at one or both transistor outputs isincreased.

Generally, RF power amplifier can be driven in a so called backed offoperation. This means that the power amplifier is operated a certainnumber level, e.g. expressed as a number of decibels (dBs), under itsmaximum output power. Backed off operation may also refer to that aninstantaneous output power is relatively low.

Referring to FIG. 1, WO03/06111 discloses a composite power amplifier 10including a first and a second power amplifier 11, 12 connected to aninput signal over an input network and to a load R_(LOAD) over an outputnetwork 13. The output network 13 includes a longer and a shortertransmission line 14, 15 for generating different phase shifts from eachpower amplifier output to the load R_(LOAD)). Each of the longer andshorter transmission lines 14, 15 connects each of the first and secondamplifiers 11, 12 to a common output at the load R_(LOAD)). In order toachieve, for this composite power amplifier 10, a widest widebandoperation, lengths of the longer and shorter transmission lines 14, 15are chosen such that the longer transmission line 14 has an electricallength of half a wavelength at a center frequency of the compositeamplifier 10, while the shorter transmission line 15 is a quarterwavelength long at the center frequency. The composite power amplifiermay be operated, typically over a 3 to 1 bandwidth, in Doherty mode, inChireix mode or in other intermediate modes between the Doherty andChireix modes. Thus, the 3 to 1 bandwidth of high efficiency is achievedby devising an output network 13 that has both suitable impedancetransformation characteristics and full power output capacity over thebandwidth. A continuous band of high efficiency amplification is thusachieved.

In FIG. 2, a simplified structure of the composite amplifier of FIG. 1is shown. The shorter and longer transmission lines are shown asbranches 21, 22 and the first and second amplifiers 11, 12 are connectedto a respective branch 21, 22. The branches 21, 22 are connected to theload R_(LOAD).

A drawback of the above mentioned composite power amplifier is that thebandwidth in which high efficiency is achieved may for some applicationsnot be sufficient.

Moreover, the above mentioned composite power amplifier may not alwaysachieve high efficiency for signals with high PAR, e.g. 10 dB.

SUMMARY

An object is to improve a power amplifier, such as the composite poweramplifier of the above mentioned kind.

According to an aspect, the object is achieved by a power amplifier,comprising a first and a second sub-amplifier, for amplification of aninput signal into an output signal. The first and second sub-amplifiersare connected to an input network for receiving the input signal at aninput port of the input network, and the first and second sub-amplifiersare connected to an output network for providing the output signal at anoutput port of the output network. The output network comprises a firsttransmission line and a second transmission line connected to the firstsub-amplifier and the second sub-amplifier, respectively. A differencein electrical length between the first and second transmission lines isan integer number of quarter-wavelengths of a center frequency of thepower amplifier.

The power amplifier further comprises a third sub-amplifier foramplification of the input signal into the output signal. The thirdsub-amplifier is connected to the input network and the output network.The output network further comprises a third transmission line connectedto the third sub-amplifier. A first electrical length includes the firsttransmission line, a second electrical length includes the secondtransmission line, and a third electrical length includes the thirdtransmission line. A longest one of the first, second and thirdelectrical lengths is at least a multiple of quarter-wavelengths of thecenter frequency.

According to another aspect, the object is achieved by a radio networknode, comprising the power amplifier.

According to a further aspect, the object is achieved by a userequipment, comprising the power amplifier.

Hence, according to some exemplifying embodiments herein, multistageamplifiers with high efficiency operation in much wider bandwidths thanthe prior art solutions are provided.

The much wider bandwidths are obtained by the output network, e.g.comprising the above mentioned first, second and third sub-amplifiers.The output network may provide multiple frequency regions, e.g. modes ofoperation, thanks to combinations of electrical length asymmetries amongthe first, second and third transmission lines.

Asymmetries in electrical length between the first, second and thirdtransmission lines, also referred to as branches, that connect to thesame point may give rise to impedance transformation in the outputnetwork. As a consequence, maintained or increased average efficiency isachieved in backed off operation.

As a result, the above mentioned object is achieved in that widerbandwidths in back off operation may be obtained.

Advantageously, some embodiments herein provide universal, verywideband, high efficiency power amplifiers. The amplifier according tosome embodiments herein may also be used without redesign or trimmingfor many different bands of operation.

Moreover, the amplifier according to some embodiments herein may bedesigned to have high efficiency, especially in backed off operation orfor high PAR input signals.

BRIEF DESCRIPTION OF THE DRAWINGS

The various aspects of embodiments disclosed herein, includingparticular features and advantages thereof, will be readily understoodfrom the following detailed description and the accompanying drawings,in which:

FIG. 1 is a schematic overview of a power amplifier according to priorart,

FIG. 2 is a schematic simplified overview of the power amplifieraccording to FIG. 1,

FIG. 3 is a schematic overview of the power amplifier according toembodiments herein,

FIG. 4 is a schematic simplified overview of the power amplifieraccording to embodiments herein,

FIGS. 5a-5d illustrate currents, voltages, and corresponding phases aswell as amplitude for each of the sub-amplifiers for input signals atrespective portion of the center frequency for an exemplifying poweramplifier,

FIG. 6 illustrates average efficiency of the power amplifiers accordingto some embodiments over a 10 to 1 bandwidth,

FIG. 7 illustrates a realization of a transmission line,

FIGS. 8a-8i illustrate exemplifying power amplifiers with threesub-amplifiers,

FIGS. 9a-9c illustrate efficiency versus frequency for some embodimentsof the power amplifier in FIG. 3,

FIG. 10 illustrates theoretical minimum efficiency for differentbandwidths for some exemplifying power amplifiers,

FIG. 11 illustrates another exemplifying embodiment of the poweramplifier,

FIGS. 12a-12d illustrate currents, voltages, and corresponding phases aswell as amplitude for each of the sub-amplifiers for input signals atrespective portion of the center frequency for another exemplifyingpower amplifier,

FIGS. 13a-13d illustrate currents, voltages, and corresponding phases aswell as amplitude for each of the sub-amplifiers for input signals atrespective portion of the center frequency for a further exemplifyingpower amplifier,

FIGS. 14a-14k illustrate further exemplifying power amplifiers,

FIGS. 15a-15h illustrate efficiency versus frequency for exemplifyingpower amplifiers according to some embodiments,

FIG. 16 illustrates theoretical minimum efficiency for differentbandwidths for some exemplifying power amplifiers,

FIGS. 17a-17d illustrate currents, voltages, and corresponding phases aswell as amplitude for each of the sub-amplifiers for input signals atrespective portion of the center frequency for a further exemplifyingpower amplifier,

FIGS. 18a and 18b illustrate efficiency versus frequency forexemplifying power amplifiers according to some embodiments,

FIG. 19 illustrates efficiency versus frequency for exemplifying poweramplifiers according to some embodiments,

FIG. 20 illustrates an exemplifying radio network node according toembodiments herein, and

FIG. 21 illustrates an exemplifying user equipment according toembodiments herein.

DETAILED DESCRIPTION

Throughout the following description similar reference numerals havebeen used to denote similar elements, units, modules, circuits, nodes,parts, items or features, when applicable. In the Figures, features thatappear in some embodiments are indicated by dashed lines.

In some of the Figures, “λ/4” denotes a quarter wavelength at a centerfrequency of a power amplifier according to some embodiment. This maymean that—at the center frequency—the “λ/4” is a quarter wavelength ofthe center frequency. “λ/4” denotes a physical length that has anelectrical length of a quarter wavelength at a center frequency.

FIG. 3 depicts an exemplifying power amplifier 100 according toembodiments herein. The power amplifier 100 comprises a first, a secondand a third sub-amplifier 111, 112, 113 which are operated to amplify aninput signal into an output signal.

The first, second and third sub-amplifiers 111, 112, 113 are connectedto an input network 120 for receiving the input signal at an input port150 of the input network 120. As an example, the input network 120 mayinclude connections (not shown) for driving of each of the first, secondand third sub-amplifiers 111, 112, 113.

Moreover, the first, second and third sub-amplifiers 111, 112, 113 areconnected to an output network 130 for providing the output signal at anoutput port 140 of the output network 130. The output network 130comprises a first transmission line 131, a second transmission line 132and a third transmission line 133 connected to the first sub-amplifier111, the second sub-amplifier 112 and the third sub-amplifier 113,respectively. A difference in electrical length between the first andsecond transmission lines 131, 132 is an integer number ofquarter-wavelengths of a center frequency of the power amplifier 100.Moreover, a further difference in electrical length between the firstand/or second transmission lines 131, 132 may be a further integernumber of quarter-wavelengths of the center frequency of the poweramplifier 100.

Hence, a first electrical length includes the first transmission line131, a second electrical length includes the second transmission line132, and a third electrical length includes the third transmission line133. A longest one of the first, second and third electrical lengths isat least a multiple of quarter-wavelengths of the center frequency.

The power amplifier 100 may be operable, e.g. efficiency of the poweramplifier 100 may be above a threshold value, down to, e.g.approximately, the center frequency divided by the multiple. In someexamples, the power amplifier 100 may be operable over a continuousbandwidth, e.g. range of frequencies, down to the center frequency.However, in some examples, the power amplifier 100 may be operable overtwo or more relatively narrow bandwidths, e.g. ranges of frequencies,where the two or more relatively narrow bandwidths may or may notinclude a lowest frequency defined as the center frequency divided bythe multiple.

In more detail, in some examples, the power amplifier 100 may beoperable somewhat lower than the center frequency divided by themultiple. Yet, it may be that in some other examples, the poweramplifier 100 is only operable down to somewhat higher than the centerfrequency divided by the multiple. Therefore, the expression “operabledown to the center frequency divided by the multiple” shall beunderstood as having a margin. The margin will for example depend onthreshold value for when the efficiency may be considered to be good.

The threshold value, e.g. for when to consider the efficiency good, maybe 60%. The threshold value is usually in the range from 30% to about70%. The lower the threshold value is set, the wider the operationalbandwidth may typically be. In further embodiments, the threshold valuemay even be outside the above mentioned range. This will be explainedfor some embodiment with reference to FIGS. 15a -15 f.

In order to maintain, for example compared to WO03/06111, efficiency ofthe power amplifier 100 at maximum output power, i.e. available outputpower, the first, second and third sub-amplifier 111, 112, 113 aredriven, across the operational bandwidth, such that the output signal isobtained by in-phase combining of respective output signals from thefirst, second and third sub-amplifier 111, 112, 113, respectively. Themaximum output power refers to maximum output power from each respectivesub-amplifier.

In some embodiments, which will be further explained with reference toFIG. 4 and FIGS. 5a -5 d, the first and second transmission lines 131,132 may be connected to a first common transmission line 135, includedin the output network 130. The first common transmission line 135 may becommon to the first and second sub-amplifiers 131, 132. In theseembodiments, the first and second electrical lengths may further includeelectrical length of the first common transmission line 135. The firstcommon transmission line 135 may be referred to as a trunk, or a firsttrunk, herein. Obviously, in embodiments in which the first commontransmission line 135 is not present, the lines that end to the rightand left of the first common transmission line 135 are connected to eachother, i.e. no break in the circuit shall occur.

In some embodiments, the power amplifier 100 may further comprise afourth sub-amplifier 114. The fourth sub-amplifier 114 may be connectedto the input network 120 and the output network 130. The output network130 may further comprise a fourth transmission line 134. Theseembodiments, a second common transmission line 136, may be devised asdescribed in more detail with reference to FIGS. 11a and 11b below.

In some embodiments, the power amplifier 100 may be operable to providethe output signal mainly supplied by the first sub-amplifier 111 in afirst mode. As an example, the first mode may be that the firstsub-amplifier 111 acts, e.g. at a first frequency, as a primarysub-amplifier. Hence, the expression “primary sub-amplifier” is used toindicate that a specific sub-amplifier makes a larger contribution tothe output signal, e.g. at the first frequency for a specific amplitude,than any other sub-amplifier make at the first frequency for thespecific amplitude. At some other amplitude, but still at the firstfrequency, any one of said any other sub-amplifier may act as primarysub-amplifier. In an example, the specific sub-amplifier may be thefirst sub-amplifier and said any other sub-amplifier may be one of thesecond and third sub.

Moreover, the power amplifier 100 may be operable to provide the outputsignal mainly supplied by the second sub-amplifier 112 in a second mode.As an example, the second mode may be that the second sub-amplifier 112acts, e.g. at a second frequency, the primary sub-amplifier.

Furthermore, the power amplifier 100 may be operable to provide theoutput signal mainly supplied by the third sub-amplifier 113 in a thirdmode.

As an example, the third mode may be that the third sub-amplifier 113acts, e.g. at a third frequency, the primary sub-amplifier. In furtherexamples, each of the first, second and third modes may be a pure ordetuned Doherty, Chireix, combined Doherty/Chireix or combinedChireix/Doherty mode.

Therefore, the power amplifier 100 may said to be a composite poweramplifier. The term composite power amplifier is herein defined asreferring to power amplifiers which may be operated in at least twodifferent modes, such as a pure or detuned Doherty, Chireix, combinedDoherty/Chireix or combined Chireix/Doherty mode.

Continuing with the example with the first, second and third frequenciesfor each of the first, second and third mode, the power amplifier may beconfigured to be driven in the first mode at the first frequency, in thesecond mode at the second frequency and in the third mode at the thirdfrequency. In some examples, when the first frequency is close to thecenter frequency divided by the multiple, the one of the first, secondand third sub-amplifiers 111, 112, 113, that is associated to thelongest one of the first, second and third electrical lengths may act asa primary amplifier. Notably, the second and third frequencies aregreater than the first frequency.

The descriptive text after FIG. 5 also explains the general behaviour,or mode of operation for different frequencies, of the power amplifier100 disclosed herein.

FIG. 4 depicts an exemplifying power amplifier 101 according embodimentsherein, in which a three-transistor amplifier is employed. This meansthat the power amplifier 101 includes the first, second and thirdsub-amplifiers 111, 112, 113. The exemplifying power amplifier has a10-to-1 bandwidth of high average efficiency. In this embodiment, thefirst and second transmission lines 131, 132 are connected to the firstcommon transmission line 135, included in the output network 130. Thefirst common transmission line 135 is common to the first and secondsub-amplifiers 131, 132.

In this embodiment, the output network 130 is configured as follows. Thefirst transmission line 131 is 2 quarter wavelengths, i.e. an electricallength of the first transmission line 131 is 2 quarter wavelengths ofthe center frequency of the power amplifier 101. The second transmissionline 132 is 1 quarter wavelength. The third transmission line 133 is 3quarter wavelengths and the first common transmission line 135, aka thefirst trunk, is 5 quarter wavelengths

Since an electrical length of any transmission line shown here isproportional to frequency and physical length, the physical lengths ofthe transmission lines are given as electrical length at centerfrequency.

The small triangles, in FIG. 4, represent sub-amplifiers, e.g. powertransistors, with accompanying wideband input match, bias and outputmatch. The electrical length of the output match is included in theelectrical lengths of the transmission lines 131, 132, 133 fromrespective sub-amplifier.

In this example, as mentioned above but now expressed somewhatdifferently, the first and second sub-amplifiers 111, 112 are connectedto the first common transmission line 135 by a half and a quarterwavelength at center frequency, respectively. This means that the firstand second transmission lines have electrical lengths of a half and aquarter wavelength, respectively. The first common transmission line 135has an electrical length of five quarter wavelengths at centerfrequency. The first common transmission line 135 is connected to theoutput port 140. The third sub-amplifier 113 is directly connected tothe output port by the third transmission line 133, which is threequarter-wavelengths at center frequency.

According to embodiments herein, the output network 130 may be built upentirely of (non-dispersive) transmission lines that are multiples of aquarter wavelength long at center frequency. In this manner, a symmetricfrequency response around center frequency may be obtained. Thanks tothe transmission lines of quarter wavelengths at center frequency thepower amplifier may be operated over a very wide bandwidth, such as 6 to1 or greater. The operation around center frequency may be a pure2-stage or multistage Doherty mode of some kind. Since the transmissionlines 131, 132, 133 are generally longer than they would be in adedicated conventional Doherty amplifier, the Doherty mode region atcenter frequency may usually be narrower in bandwidth in the poweramplifiers according to embodiments herein, even though the totalhigh-efficiency bandwidth is far greater than that of a conventionalDoherty amplifier.

The wideband operation, i.e. amplifying a relatively narrowband signalat any frequency in a wide band, instead relies on using many othermodes of operation. The operation modes vary across the bandwidth, oroperational bandwidth, and may include pure or detuned Chireix-Doherty,Doherty-Chireix and Doherty modes and transitional modes between thesemodes. The different modes of operation at different frequencies usuallyrequire differently shaped drive signals as is exemplified by FIGS.5a-5c and other similar sets of Figures.

FIGS. 5a-5c show operation of the exemplary power amplifier 101 of FIG.4 at various frequencies within one half of the 10-to-1 bandwidth. Theother half is a mirror image; even symmetry for the amplitudes and oddfor the phases. For the 10-to-1 bandwidth to be centered at 1, it mustgo from about 0.18 to 2-0.18 (=1.8), so the lowest frequency supportedis 0.18 times the center frequency. In these Figures, the firstsub-amplifier 111 is represented by a dotted line, the secondsub-amplifier 112 is represented by a solid line and the thirdsub-amplifier 113 is represented by a dashed line.

Now referring in detail to FIG. 5a , which comprises six smaller FIGS.5a :1-5 a:6, each of these smaller Figures will be described. In orderto understand the context of these Figures, FIG. 5a :6 is describedfirst.

Thus, FIG. 5a :6 illustrates, beginning at the top of FIG. 5a :6, thethird transmission line 133 with an electrical length of 0.14λ at 0.18times the center frequency f_(c)., since 0.18*0.75=0.135=−0.14.Similarly, the first and second transmission lines 131, 132 and thefirst common transmission line 135 have electrical lengths of 0.045λ,0.091λ and 0.23λ, respectively for this frequency, i.e. at 0.18*f_(c).

From FIG. 5a :1, it may be seen that the second sub-amplifier 112 isoperated as a primary sub-amplifier at this frequency and for amplitudesup to about 0.5. This means that the second sub-amplifier 112 outputs acurrent that is greater than any respective currents from the first andthird sub-amplifiers 111, 113. Moreover, it may also be seen that thethird sub-amplifier 113 is not contributing at all up to aboutamplitudes of 0.6.

FIG. 5a :2 shows RF voltage as a function of amplitude when operatingthe power amplifier 101 at 0.18*f_(c). This Figure shows, e.g., that allsub-amplifiers 111, 112, 113 are saturated for amplitudes above about0.5. Moreover, the second sub-amplifier 112 increases voltage fasterthan the first and third sub-amplifiers 111, 113.

FIG. 5a :3 shows total efficiency for all sub-amplifiers 111, 112, 113as a function of amplitude when operating the power amplifier 101 at0.18*f_(c). This Figure shows, e.g., that total efficiency increaseslinearly up to an amplitude of about 0.5.

FIG. 5a :4 shows RF current phase as a function of amplitude whenoperating the power amplifier 101 at 0.18*f_(c). This Figure shows,e.g., that the second sub-amplifier has the highest current phase overall amplitudes. This depends on that the second sub-amplifier has thelongest electrical length towards the output port and the phase of thecurrent compensates for this. A positive phase means ahead, or before,in time. If all phases are greater than 2*pi (2*3.1415 . . . ), it canbe reduced with 2*pi in a narrowband perspective.

FIG. 5a :5 shows RF voltage phase as a function of amplitude whenoperating the power amplifier 101 at 0.18*f_(c). This Figure shows,e.g., that the second sub-amplifier has the highest voltage phase overall amplitudes. Similarly to FIG. 5a :4, this depends on that the secondsub-amplifier has the longest electrical length towards the output portand the phase of the voltage compensates for this. A positive phasemeans ahead, or before, in time. If all phases are greater than 2*pi(2*3.1415 . . . ), it can be reduced with 2*pi in a narrowbandperspective.

Similar observations may be made for each of the first, second and thirdsub-amplifiers 111, 112, 113 while studying FIGS. 5b, 5c and 5d . As anexample, with reference to FIG. 5b :1, the second sub-amplifier 112 isoperated as primary sub-amplifier at frequency 0.39*f_(c) for amplitudesabove about 0.3. As another example, with reference to FIG. 5c :2,saturation for each of the second, first and third sub-amplifiers isreached at amplitudes of about 0.2, 0.4 and 0.9, respectively, atfrequency 0.59*f_(c). As a further example, with reference to FIG. 5d:3, total efficiency increases linearly up to an amplitude of about 0.35for frequency of 0.8*f_(c).

As can be observed above with reference to FIGS. 5a -5 d, the RF outputcurrents of the transistors, referred to as sub-amplifiers above, andthe RF voltages are thus as follows, from low to high amplitudes:

1) One transistor delivers all RF current, linearly increasing withamplitude and with a constant phase relative to the output. All voltagesare below saturation and breakdown limits. Efficiency is in this regionproportional to the amplitude and to the trans-impedance from the driventransistor to the output. This region continues until one transistorvoltage reaches a limit.

2) One transistor is voltage-limited. Two transistors deliver RFcurrent. Their phases relative to the output generally change withamplitude. This continues until two transistors are voltage limited.

3) Two transistors are voltage limited, often similar to what is called“outphasing” in a symmetric 2-transistor Chireix amplifier, withincreasing RF current amplitudes. This continues until it is moreefficient to start a third transistor, not necessarily where thepossibility of outphasing ends.

4) Two transistors voltage limited with a third transistor alsodelivering RF current, and not voltage limited.

5), and so on . . .

In FIG. 6, a diagram over efficiency versus frequency is illustrated.From about 0.18 to 1.8 relative to the center frequency, i.e. from 0.5to 5 GHz, a resulting average efficiency with class B operation of thesub-amplifiers for use with a narrowband signal with a 7 dB Rayleighamplitude distribution is plotted in the Figure. An average efficiencyfor a narrowband signal of over 60% is achieved in the 10-to-1bandwidth, i.e. from 0.5 GHz to 5 GHz in this example.

The electrical length of an output matching network for eachsub-amplifier may be different depending on the frequency range to becovered. Each of the first, second and third transmission lines 131,132, 133 of the output network includes a respective output matchingnetwork for each sub-amplifier. For wideband operation towards highfrequencies, the output network is largely determined by the capacitanceof the output node, Cds (ds=drain-source), which is “absorbed” into asuitable network. Although it is usually called a “matching” network,impedance transformation is not the primary objective, and usually morewideband operation is possible if very little transformation is done inthis part, instead transforming the load to a value that is compatiblewith the largely untransformed sum of admittances.

Now turning to FIG. 7, an arrangement of components called a pi-networkis shown. The arrangement of components has an electrical length ofabout a quarter wavelength at the upper frequency limit, and at centerfrequency about an eight of a wavelength, and corresponds quite well toa fixed physical length. The electrical lengths of the output networkare deducted from the transmission line lengths of the power amplifier101 of FIG. 4. The remaining of the transmission line length can bebuilt from one or more further pi-networks, transmission lines(“distributed”), or semi-lumped varieties with both lumped anddistributed elements. For lower frequency operation, other pi-matchdimensioning or a simple L-match may be used, and sometimes nocompensation at all is needed

The class B assumption requires low-impedance termination of harmonicstwo and higher at the output of a sub-amplifier, e.g. drain or collectorof a transisor. This is possible roughly above center frequency, for thelower half the harmonics fall inside or too close to the supportedfundamental band. For wideband operation including the lower frequencyrange operation similar to class B, but without the harmonic terminationcan be used.

In some cases it is sufficient to simply terminate the harmonicsresistively for the lower part of the efficient bandwidth. Resistivetermination outside the band is achieved by using a wideband isolatorbefore the selected (or tuneable) channel/band filter. All the poweroutside the band is reflected by the filter and terminated in thebackwards direction by the isolator.

Another method is to use a diplexed load for the harmonics. In this casea high-pass path to a resistor (dummy load) is provided. Since thesecond harmonic is quite far from the fundamental band, this filter canbe simple and cheap. Both these methods terminate the harmonics outsidethe output network, so reflections within the output network can stillaffect efficiency. Tuneable tank circuits, or resonator, at thetransistor outputs are of course also possible.

A wideband method to get high efficiency and low harmonic contentdirectly at the sub-amplifier is to use a push-pull arrangement of classB driven transistors. The term “push-pull” has its conventional meaningthat is known within the field of power amplifiers. A single-ended,simpler but less efficient, wideband alternative is to use class A withdynamically amplitude-following gate bias to eliminate excess DCcurrent.

FIGS. 8a-8i illustrate schematically a number of differentconfigurations of the output network 130 for the power amplifier 100comprising three sub-amplifiers 111, 112, 113.

In these Figures, the following nomenclature is used. Referring to FIG.8a , the reference numerals 131, 132, 133 denote the first, second andthird transmission lines as is the case also in FIG. 3. Also as in FIG.3, the reference numeral 135 denotes the first common transmission line.Additionally, a character ‘1’ means an electrical length is one quarterwavelength for the transmission line at which an arrow next to thecharacter points. Similarly, a character ‘2’ means an electrical lengthis one quarter wavelength for the transmission line at which an arrownext to the character points, etc.

Hence, as indicated for the configuration in FIG. 8a , the four numbersindicate the electrical lengths at center frequency in quarterwavelengths. The first three numbers are the lengths of the transmissionlines 131, 132, 133 originating from the three sub-amplifiers 111, 112,113, and the fourth number is the length of the first commontransmission line 135 from the junction of the first and secondtransmission lines 131, 132 from sub-amplifiers 111, 112 to the output140. Therefore, configuration of the output network 130, shown in FIG.8a , is denoted “1 2 2 1”. The third sub-amplifier's 113 thirdtransmission line 133 is then connected directly to the output port 140as for all embodiments including three sub-amplifiers.

In FIG. 8b , the first transmission line 131 has an electrical length ofone, 1, quarter wavelength, the second transmission line 132 has anelectrical length of two, 2, quarter wavelengths, the third transmissionline 133 has an electrical length of one, 1, quarter wavelengths, andthe first common transmission line 135 has an electrical length of zero,0, quarter wavelengths. Thus, the nomenclature is “1 2 0 1”.

In FIG. 8c , the first transmission line 131 has an electrical length of0 quarter wavelengths, the second transmission line 132 has anelectrical length of 1 quarter wavelengths, the third transmission line133 has an electrical length of 3 quarter wavelengths, and the firstcommon transmission line 135 has an electrical length of 1 quarterwavelengths. Thus, the nomenclature is “0 1 3 1”.

In FIG. 8d , the first transmission line 131 has an electrical length of1 quarter wavelengths, the second transmission line 132 has anelectrical length of 4 quarter wavelengths, the third transmission line133 has an electrical length of 2 quarter wavelengths, and the firstcommon transmission line 135 has an electrical length of 3 quarterwavelengths. Thus, the nomenclature is “1 4 2 3”.

In FIG. 8e , the first transmission line 131 has an electrical length of1 quarter wavelengths, the second transmission line 132 has anelectrical length of 2 quarter wavelengths, the third transmission line133 has an electrical length of 4 quarter wavelengths, and the firstcommon transmission line 135 has an electrical length of 0 quarterwavelengths. Thus, the nomenclature is “1 2 4 0”.

In FIG. 8f , the first transmission line 131 has an electrical length of0 quarter wavelengths, the second transmission line 132 has anelectrical length of 1 quarter wavelengths, the third transmission line133 has an electrical length of 2 quarter wavelengths, and the firstcommon transmission line 135 has an electrical length of 3 quarterwavelengths. Thus, the nomenclature is “0 1 2 3”.

In FIG. 8g , the first transmission line 131 has an electrical length of1 quarter wavelengths, the second transmission line 132 has anelectrical length of 2 quarter wavelengths, the third transmission line133 has an electrical length of 2 quarter wavelengths, and the firstcommon transmission line 135 has an electrical length of 2 quarterwavelengths. Thus, the nomenclature is “1 2 2 2”.

In FIG. 8h , the first transmission line 131 has an electrical length of3 quarter wavelengths, the second transmission line 132 has anelectrical length of 4 quarter wavelengths, the third transmission line133 has an electrical length of 2 quarter wavelengths, and the firstcommon transmission line 135 has an electrical length of 1 quarterwavelengths. Thus, the nomenclature is “3 4 2 1”.

In FIG. 8i , the first transmission line 131 has an electrical length of1 quarter wavelengths, the second transmission line 132 has anelectrical length of 2 quarter wavelengths, the third transmission line133 has an electrical length of 3 quarter wavelengths, and the firstcommon transmission line 135 has an electrical length of 5 quarterwavelengths. Thus, the nomenclature is “1 2 3 5”.

FIG. 9a-9c illustrate diagrams in which efficiency versus frequency forsome exemplifying power amplifiers is plotted. The configuration of theoutput network is noted—using the nomenclature as above—in each diagram,directly above the axis of frequency (horizontal axis). For theseexemplifying power amplifiers minimum average class B efficiency withinthe bandwidth is relatively higher than other (not shown) testedconfigurations of the output network. The power amplifier receives asignal with a 7 dB Rayleigh distributed amplitude as noted above eachdiagram. Also above each diagram, an operational bandwidth of the poweramplifier is noted. Moreover, the threshold value for efficiency isnoted in each diagram, usually under the curve but in the upper rightcorner of the plot area.

In FIG. 9a , efficiency for an exemplifying power amplifier with anoutput network 130 configured as “1 2 3 5” is shown. In this example,the efficiency is at least 62% for the operational bandwidth of 6.1to 1. Reference is made to FIG. 8 i.

In FIG. 9b , efficiency for an exemplifying power amplifier with anoutput network 130 configured as “1 4 2 3” is shown. Reference is madeto FIG. 8 d.

In FIG. 9c , efficiency for an exemplifying power amplifier with anoutput network 130 configured as “2 8 4 1” is shown. Similarly to theexamples of FIG. 8, this means that the first transmission line 131 hasan electrical length of 2 quarter wavelengths, the second transmissionline 132 has an electrical length of 8 quarter wavelengths, the thirdtransmission line 133 has an electrical length of 4 quarter wavelengths,and the first common transmission line 135 has an electrical length of 1quarter wavelengths. Thus, the nomenclature is “2 8 4 1”.

Turning to FIG. 10 theoretical minimum efficiency for differentbandwidths in class B mode of the power amplifier for a number ofdifferent structures, illustrating that the output networks whichinclude the first common transmission line 135 are generally moreefficient, but a configuration of the output network according to “1 24” (without trunk) to the common output is relatively good up to a 7:1bandwidth.

In some embodiments, higher order configurations of the output network130 are employed. Due to the higher number of electrical lengthcombinations in these embodiments, longer transmission lines may beused. In this manner, wider bandwidth with high efficiency may beobtained. Alternatively or additionally, the output network may beconfigured to obtain high efficiency over a somewhat smaller bandwidthbut for signals with larger PAR values.

In FIG. 11a , an embodiment of the power amplifier 100 is shown. In thisexample, the power amplifier 100 further comprises the fourthsub-amplifier 114. The fourth sub-amplifier 114 is connected to theinput network 120 and the output network 130. The output network 130further comprises the fourth transmission line 134.

In some examples, the fourth transmission line 134 may be connecteddirectly to the output port 140, as illustrated in FIG. 11a and byconnector 160 in FIG. 3. Moreover, a fourth electrical length mayinclude electrical length of the fourth transmission line 134. Since thelines to the sub-amplifiers branch out from a single trunk line, e.g.the second common transmission line 136, at different points thisconfiguration of the output network is referred as “serial branched”,denoted S in the FIG. 11a . In contrast, the configuration of the outputnetwork 130, as shown in FIG. 11b below, is referred to as “parallelbranched”, denoted P in FIG. 11 b.

In these embodiments, the third and fourth sub-amplifier 133, 134 may beconnected to the second common transmission line 136, included in theoutput network 130. Obviously, in embodiments in which the second commontransmission line 136 is not present, the lines that end to the rightand left of the second common transmission line 136 are connected toeach other, i.e. no break in the circuit shall occur. The second commontransmission line 136, or a second trunk, may be common to the third andfourth sub-amplifiers 133, 134, as shown in FIG. 11b . However, it shallbe noted that in some examples the second common transmission line 136is connected the third transmission line 133, but not to the fourthtransmission line 134, as already shown in FIG. 11a . The notion‘common’ is due to that the first common transmission line 135 isconnected to the second common transmission line 136, which make thesecond common transmission line 136 indirectly common to the first,second and third sub-amplifiers 111, 112, 113. Therefore, the thirdelectrical length may include electrical length of the second commontransmission line 136. For the fourth electrical length it may be thatelectrical length of the second common transmission line is not includesas in FIG. 11a , while in other examples as in FIG. 11b the fourthelectrical length includes electrical length of the second commontransmission line 136.

With reference to FIGS. 12a -12 d, the power amplifier of FIG. 11a isshown to have high average efficiency in a 12-to-1 bandwidth. Asmentioned, the electrical lengths of the transmission lines from thesub-amplifiers at center frequency are (from the top down) 1, 2, 5, and7 quarter wavelengths, and the lengths of the trunk line segmentsbetween the connection points are one quarter wavelength each.

FIGS. 12a-12c show operation of the power amplifier of FIG. 11a atvarious frequencies within one half of the 12-to-1 bandwidth, startingat 0.15 of the center frequency. As with the previous example of a3-stage amplifier, i.e. the power amplifier comprises threesub-amplifiers, this 4-stage amplifier uses different combinations ofoperating modes at different frequencies, and achieves good efficiencycurves over the whole bandwidth. Similar observations as for FIGS. 5a-5dmay be made here without further elaboration in detail.

Referring to FIGS. 13a-13d , operation of another 4-stage poweramplifier is illustrated. In this example, the 4-stage power amplifier,comprising the first, second, third and fourth sub-amplifiers 111, 112,113, 114, employs an output network, which has a configuration that isshown to have high average efficiency in an 8-to-1 bandwidth.

In this exemplifying output network 130, also shown in FIG. 11b , theelectrical lengths of the transmission lines from the sub-amplifiers atcenter frequency are (from the top down) 2, 3, 1 and 2 quarterwavelengths, and the lengths of the two trunk lines, e.g. the first andsecond common transmission lines 135, 136, that both connect directly tothe load, that are 1 and 4 quarter wavelengths each. Since the first andsecond trunk lines branch out from (or, looking in the other direction,come together to) the same point (the load), this type of configurationis referred to as “parallel branched” herein. Reference is made to theschematic configurations shown in the sixth smaller Figure of FIGS.13a-d , e.g 13 a:6, 13 b:6, etc, for each respective frequency.

Similar observations as for FIGS. 5a-5d may be made here without furtherelaboration in detail.

In FIGS. 14a -14 k, further exemplifying output networks 130 areillustrated schematically. In these exemplifying power amplifiers arealso referred to as 4-stage amplifiers since the power amplifiersinclude four sub-amplifiers. The output networks may be configured inserial or parallel manners of branching.

For some of the power amplifiers of FIGS. 14a-14k efficiency versusfrequency is plotted in FIGS. 15a -15 h. Similarly to FIGS. 9a -9 c,configuration of the output network, operational bandwidth, PAR ofsignal and efficiency is shown in the diagrams. Therefore, reference ismade to the diagrams themselves in order to make observations. FIGS.15a-g show diagrams for 7 dB PAR Rayleigh distributed amplitude, whileFIGS. 15e-h show diagrams for 10 dB PAR Rayleigh distributed amplitude.

This means that some embodiments of the power amplifier may haveincreased backed off operation, e.g. higher number of dBs, i.e. 3 dBs(10-7) as in the examples above, with high efficiency.

FIG. 16 shows theoretical minimum efficiency within different bandwidthsin class B mode for some configurations of the output network 130. Thebranched output networks (serial, parallel and single trunk) performbest. The un-branched network with 1, 2, 4, and 8 quarter wavelengths tothe common output at center frequency is relatively good for the widestbandwidths, but far behind the branched configurations at the lowerbandwidths.

Networks using only line lengths that are multiples of a quarterwavelength have a periodically repeating frequency response pattern. Thefirst instance of a higher mode, i.e. a mode with higher efficiency,occurs at three times the first mode center frequency. The bandwidth ofthe wideband amplifiers described above goes very close to twice thecenter frequency, so the repeating pattern will have just a smallunsupported region before the higher mode starts. Using the first andsecond modes of the 12-to-1 bandwidth example, the unsupported region is2*0.15 in the original frequency scale, with the total bandwidth goingfrom 0.15 to 4-0.15. This gives a 25-to-1 bandwidth with a 15% relativebandwidth around center frequency (now at two times the original centerfrequency) unsupported.

The equivalent of placing the center frequency at two times the originalcenter frequency is to build the output network only from lines that aremultiples of a half wavelength. This can be advantageous if there is noneed for operation in a middle region, since the efficiency in the twosupported regions is higher for the same total (lowest to highest)bandwidth. The technique can trivially be extended to responses havingthree (using only multiples of three quarter wavelengths at centerfrequency), four or more regions. The only requirement is that the linesare built only from multiples of some specific line length.

In the previous examples, very wideband operation is achieved. In thosecases, a symmetric or close to symmetric frequency response, obtained byusing lines with lengths that are multiples of a quarter wavelength atcenter frequency, generally gives the best results. For less widebandoperation, higher efficiency can sometimes be achieved by using othertransmission line lengths than multiples of quarter-wavelengths of thecenter frequency. As an example, operation of a 3-stage power amplifierthat achieves high average efficiency for signals with large PAR in a2.5 to 1 bandwidth is shown in FIGS. 17a -17 d. The line lengths are inthis case about 0.21, 0.32 and 0.52 wavelengths at center frequency, andare all connected directly to the output, i.e. no first commontransmission line is employed. Similar observations as for FIGS. 5a-5dmay be made here without further elaboration in detail.

The class B efficiency for a signal with 7 dB PAR Rayleigh distributedamplitude is 70% or higher in the 2.5-to-1 frequency range, as shown inFIG. 18a .

The class B efficiency for a signal with 10 dB PAR Rayleigh distributedamplitude is 62% or higher in the 2.5-to-1 frequency range, as shown inFIG. 18b .

Hence, these embodiments of the power amplifier have increasedefficiency, but not increased bandwidth, in backed off operation.

In the embodiments described above, the sub-amplifiers 111, 112, 113,114 may have the same size.

However, in some embodiments different sizes for the differentamplifiers may be used. For example, one sub-amplifier may have twicethe size of the two other sub-amplifiers in case of a 3-stage poweramplifier. It is also possible to make a configuration with a trunk linefrom the connection point of two of the lines from the sub-amplifiers tothe output. An example of both these features is a power amplifier inwhich sub-amplifiers 1 and 3 have half the size of amplifier 2 (and thelines from sub-amplifiers 1 and 3 consequentially having twice thecharacteristic impedance of the line from sub-amplifier 2).Sub-amplifiers 1 and 2 are connected via lines of length 0.22 and 0.49wavelengths (at center frequency) to a trunk line of 0.05 wavelengths,which trunk line is connected to the output. Sub-amplifier 3 is coupledvia a line that is 0.32 wavelengths at center frequency. The efficiencyin class B mode is better than 63% over a frequency range of 2.5 (oreven 2.6) to 1 (0.56 to 1.44), as shown in FIG. 19.

Hence, expressed somewhat differently, each of the first and thirdsub-amplifier 111, 113 may have half of a size of the secondsub-amplifier 112. Moreover, the first and second transmission lines131, 132 have electrical lengths of 0.22 wavelengths and 0.49wavelengths, respectively and the first common transmission line 135 haselectrical length of 0.05 wavelengths. The third transmission line 133has electrical length of 0.32 wavelengths. All wavelengths here arerelative the center frequency of the power amplifier.

FIG. 20 shows an exemplifying radio network node 200.

As used herein, the term “radio network node” may refer to is a piece ofequipment that facilitates wireless communication between user equipment(UE) and a network. Accordingly, the term “radio network node” may referto a Base Station (BS), a Base Transceiver Station (BTS), a Radio BaseStation (RBS), a NodeB in so called Third Generation (3G) networks,evolved Node B, eNodeB or eNB in Long Term Evolution (LTE) networks, orthe like. In UMTS Terrestrial Radio Access Network (UTRAN) networks,where UTMS is short for Universal Mobile Telecommunications System, theterm “radio network node” may also refer to a Radio Network Controller.Furthermore, in Global System for Mobile Communications (GSM) EDGE RadioAccess Network (GERAN), where EDGE is short for Enhanced Data rates forGSM Evolution, the term “radio network node” may also refer to a BaseStation Controller (BSC).

The radio network node 200 comprises a power amplifier 210 according tothe embodiments described above.

Furthermore, the radio network node 200 may comprise a processingcircuit 220 and/or a memory 230.

As used herein, the term “processing circuit” may be a processing unit,a processor, an application specific integrated circuit (ASIC), afield-programmable gate array (FPGA) or the like. As an example, aprocessor, an ASIC, an FPGA or the like may comprise one or moreprocessor kernels. In some examples, the processing circuit may beembodied by a software or hardware module. Any such module may be adetermining means, estimating means, capturing means, associating means,comparing means, identification means, selecting means, receiving means,transmitting means or the like as disclosed herein. As an example, theexpression “means” may be a unit, such as a determining unit, selectingunit, etc.

As used herein, the term “memory” may refer to a hard disk, a magneticstorage medium, a portable computer diskette or disc, flash memory,random access memory (RAM) or the like. Furthermore, the term “memory”may refer to an internal register memory of a processor or the like.

The radio network node 200 may further comprise additional transceivercircuitry (not shown) for facilitating transmission and reception ofdata, e.g. in the form of radio signals.

FIG. 21 shows an exemplifying user equipment 300.

As used herein, the term “user equipment” may refer to a mobile phone, acellular phone, a Personal Digital Assistant (PDA) equipped with radiocommunication capabilities, a smartphone, a laptop or personal computer(PC) equipped with an internal or external mobile broadband modem, atablet PC with radio communication capabilities, a portable electronicradio communication device, a sensor device equipped with radiocommunication capabilities or the like. The sensor may be any kind ofweather sensor, such as wind, temperature, air pressure, humidity etc.As further examples, the sensor may be a light sensor, an electronicswitch, a microphone, a loudspeaker, a camera sensor etc.

The user equipment 300 comprises a power amplifier 310 according to theembodiments described above.

Furthermore, the user equipment 300 may comprise a processing circuit320 and/or a memory 330. The means of the terms “processing circuit” and“memory” as explained above applies also for the user equipment 300.

The user equipment 300 may further comprise additional transceivercircuitry (not shown) for facilitating transmission and reception ofdata, e.g. in the form of radio signals.

As used herein, the terms “number”, “value” may be any kind of digit,such as binary, real, imaginary or rational number or the like.Moreover, “number”, “value” may be one or more characters, such as aletter or a string of letters. “number”, “value” may also be representedby a bit string.

As used herein, the expression “in some embodiments” has been used toindicate that the features of the embodiment described may be combinedwith any other embodiment disclosed herein.

Even though embodiments of the various aspects have been described, manydifferent alterations, modifications and the like thereof will becomeapparent for those skilled in the art. The described embodiments aretherefore not intended to limit the scope of the present disclosure.

1. A power amplifier comprising a first and a second sub-amplifier foramplification of an input signal into an output signal, wherein thefirst and second sub-amplifiers are connected to an input network forreceiving the input signal at an input port of the input network, andthe first and second sub-amplifiers are connected to an output networkfor providing the output signal at an output port of the output network,wherein the output network comprises a first transmission line and asecond transmission line connected to the first sub-amplifier and thesecond sub-amplifier, respectively, wherein a difference in electricallength between the first and second transmission lines is an integernumber of quarter-wavelengths of a center frequency of the poweramplifier, wherein the power amplifier is characterized by furthercomprising: a third sub-amplifier for amplification of the input signalinto the output signal, wherein the third sub-amplifier is connected tothe input network and the output network, wherein the output networkfurther comprises a third transmission line connected to the thirdsub-amplifier, wherein a first electrical length includes the firsttransmission line, a second electrical length includes the secondtransmission line, and a third electrical length includes the thirdtransmission line, and wherein a longest one of the first, second andthird electrical lengths is at least a multiple of quarter-wavelengthsof the center frequency.
 2. The power amplifier according to claim 1,wherein the power amplifier is operable down to the center frequencydivided by the multiple.
 3. The power amplifier according to claim 1,wherein the first and second transmission lines are connected to a firstcommon transmission line, included in the output network, wherein thefirst common transmission line is common to the first and secondsub-amplifiers.
 4. The power amplifier according to claim 1, furthercomprising a fourth sub-amplifier, wherein the fourth sub-amplifier isconnected to the input network and the output network, wherein theoutput network further comprises a fourth transmission line.
 5. Thepower amplifier according to claim 4, wherein the third and fourthsub-amplifier are connected to a second common transmission line,included in the output network, wherein the second common transmissionline is common to the third and fourth sub-amplifiers.
 6. The poweramplifier according to claim 1, wherein the power amplifier is operableto: provide the output signal mainly supplied by the first sub-amplifierin a first mode; provide the output signal mainly supplied by the secondsub-amplifier in a second mode; and provide the output signal mainlysupplied by the third sub-amplifier in a third mode.
 7. The poweramplifier according to claim 1, wherein the power amplifier is acomposite power amplifier.
 8. A radio network node comprising: a poweramplifier comprising a first and a second sub-amplifier foramplification of an input signal into an output signal, wherein thefirst and second sub-amplifiers are connected to an input network forreceiving the input signal at an input port of the input network, andthe first and second sub-amplifiers are connected to an output networkfor providing the output signal at an output port of the output network,wherein the output network comprises a first transmission line and asecond transmission line connected to the first sub-amplifier and thesecond sub-amplifier, respectively, wherein a difference in electricallength between the first and second transmission lines is an integernumber of quarter-wavelengths of a center frequency of the poweramplifier, wherein the power amplifier is characterized by furthercomprising: a third sub-amplifier for amplification of the input signalinto the output signal, wherein the third sub-amplifier is connected tothe input network and the output network, wherein the output networkfurther comprises a third transmission line connected to the thirdsub-amplifier, wherein a first electrical length includes the firsttransmission line, a second electrical length includes the secondtransmission line, and a third electrical length includes the thirdtransmission line, and wherein a longest one of the first, second andthird electrical lengths is at least a multiple of quarter-wavelengthsof the center frequency.
 9. A user equipment comprising: a poweramplifier comprising a first and a second sub-amplifier foramplification of an input signal into an output signal, wherein thefirst and second sub-amplifiers are connected to an input network forreceiving the input signal at an input port of the input network, andthe first and second sub-amplifiers are connected to an output networkfor providing the output signal at an output port of the output network,wherein the output network comprises a first transmission line and asecond transmission line connected to the first sub-amplifier and thesecond sub-amplifier, respectively, wherein a difference in electricallength between the first and second transmission lines is an integernumber of quarter-wavelengths of a center frequency of the poweramplifier, wherein the power amplifier is characterized by furthercomprising: a third sub-amplifier for amplification of the input signalinto the output signal, wherein the third sub-amplifier is connected tothe input network and the output network, wherein the output networkfurther comprises a third transmission line connected to the thirdsub-amplifier, wherein a first electrical length includes the firsttransmission line, a second electrical length includes the secondtransmission line, and a third electrical length includes the thirdtransmission line, and wherein a longest one of the first, second andthird electrical lengths is at least a multiple of quarter-wavelengthsof the center frequency.